Many systems utilize components or transducers that provide information in the form of an analog signal. One example is an analog output temperature sensor, which generates a linear output voltage directly proportional to measured temperature. An output signal in analog form is often desirable because only one wire (and ground) is required to transmit information to other devices in the system. The circuit complexity in analog output devices is also lower than equivalent functions fitted with digital output encoding, thereby lowering device cost. In most cases, however, analog signals are required to be converted to a digital format to be assimilated by system computing hardware. This is accomplished with an analog-to-digital converter (ADC). In cases where several analog signals must be converted, a single ADC with an inexpensive input multiplexer (MUX) is used (instead of separate ADCs for each signal). The MUX is a "many-to-1" selector switch that connects only one of the input signals to the ADC at a time. This allows all input signals to be sequentially converted with a single ADC, saving cost and simplifying system hardware.
In certain applications, the number of analog signals requiring conversion exceeds the input capacity of the MUX. One example is a notebook PC where temperature in four different locations must be measured, plus main and back-up battery voltages. This problem can be remedied in four ways: 1) obtain a MUX having a larger input capacity; 2) add a second MUX; 3) modify the system architecture to replace the analog output devices with digital ones; or 4) add another ADC. Each of these approaches has inherent disadvantages. Higher input capacity MUXs have a greater number of pins and, therefore, a larger package size and greater cost. In many system applications, both the MUX and ADC are contained on board a microcontroller. Many microcontrollers have a MUX with a maximum of four inputs, and versions with a larger MUX may not exist. Adding a second MUX adds cost and adversely impacts board space. Replacing the analog output devices with digital output types adds significant cost. For example, a temperature sensor with a two wire, serial output can cost as much as three times that of the same temperature sensor with an analog output. Adding a second ADC adds significant cost and complicates the system address decoding circuitry in that unique address space must be created for the second ADC. In addition, all four of these approaches require a separate circuit board trace from the analog signal source to the MUX input. This complicates circuit board layout and potentially adds electrical noise to the analog signals.
What is needed is a more simple and inexpensive technique for transmitting data from a plurality of analog signal generators to a receiving circuit.